This invention is related to that described and claimed in copending, commonly owned application Ser. No. 08/066,887 filed 25 May 1993 by Whittenberger and entitled "Combined Electrically Heatable Converter Body", the disclosure of which application is incorporated herein by reference thereto. The invention will be described in connection with embodiments especially adapted for use in exhaust lines from various types of engines, e.g., internal combustion engines of the spark ignited or compression ignited types, stationary or mobile, or gas turbine engines. It will be understood, however, that the converters of the present invention may be used to effect various chemical reactions, particularly those occurring in fluid streams, especially gas streams, which reactions are catalyzed or uncatalyzed.
Turning now to converters especially useful in exhaust lines extending from internal combustion engines, e.g., those used in automotive vehicles, the purpose of such catalytic converters is to convert pollutant materials present in the exhaust stream, e.g., carbon monoxide, unburned hydrocarbons, nitrogen oxides, ozone, etc., to carbon dioxide, nitrogen and water prior to discharge into the atmosphere. Conventional automotive catalytic converters utilize an oval or circular cross-section ceramic honeycomb monolith having square or triangular straight-through openings or cells, with catalyst deposited on the walls of the cells; catalyst coated refractory metal oxide beads; or corrugated thin metal foil, multicelled honeycomb monolith, having a refractory metal oxide coating and a catalyst carried on said coating and supported on the surfaces of the cells. The catalyst is normally a noble metal, e.g., platinum, palladium, rhodium, ruthenium, or a mixture of two or more of such metals. Zeolite coatings may also be used for the adsorption and desorption of pollutants to aid in their removal. The catalyst catalyzes a chemical reaction, mainly oxidation, whereby the pollutant is converted to a harmless by-product which passes through the exhaust system to the atmosphere.
However, conversion to such harmless by-products is not efficient initially when the exhaust gases are relatively cold, e.g., at cold engine start. To be effective at high conversion rate, the catalyst and the surface of the converter which the exhaust gases contact must be at or above a minimum temperature, e.g., 390.degree. F. for carbon monoxide, 570.degree. F. for volatile organic compounds (VOC) and about 900.degree. F. for methane or natural gas. Otherwise, conversion to harmless by-products is poor and cold start pollution of the atmosphere is high. It has been estimated that as much as 80% of the atmospheric pollution caused by vehicles, even though equipped with conventional non-electrically heated catalytic converters, occurs in the first two minutes of operation of the engine from cold start. Once the exhaust system has reached its normal operating temperature, a non-electrically heated catalytic converter is optimally effective. Hence, it is necessary for the relatively cold exhaust gases to make contact with hot catalyst so as to effect satisfactory conversion. Compression ignited engines, spark ignited engines, reactors in gas turbines, small bore engines, such as used in lawn mowers, trimmers, boat engines and the like have this need.
To achieve initial heating of the catalyst at engine start-up, there is conveniently provided an electrically heatable catalytic converter unit, preferably one formed of a thin metal honeycomb monolith. This monolith may be formed of spaced flat thin metal strips, straight,through corrugated thin metal strips, pattern corrugated thin metal strips, e.g., herringbone or chevron corrugated thin metal strips, or variable pitch corrugated thin metal strips (such as disclosed in U.S. Pat. No. 4,810,588 dated 7 Mar. 1989 to Bullock et al) or a combination thereof. This monolith is connected to a 12 volt to 108 volt or higher, AC or DC supply, single or multiphase, preferably at the time of engine start-up and afterwards to elevate the catalyst to and maintain the catalyst at at least 650.degree. F. plus or minus 30.degree. F. Alternatively, power may also be supplied for a few seconds prior to engine start-up.
Catalytic converters containing a corrugated thin metal (stainless steel) monolith have been known since at least the early 1970's. See Kitzner U.S. Pat. Nos. 3,768,982 and 3,770,389 each dated 30 Oct. 1973. More recently corrugated thin metal monoliths have been disclosed in U.S. Pat. No. 4,711,009 dated 8 Dec. 1987 to Cornelison et al; U.S. Pat. Nos. 4,152,302 dated 1 May 1979, 4,273,681 dated 16 Jun. 1981, 4,282,186 dated 4 Aug. 1981, 4,381,590 dated 3 May 1983, 4,400,860 dated 30 Aug. 1983, 4,519,120 dated 28 May 1985, 4,521,947 dated 11 Jun. 1985, 4,647,435 dated 3 Mar. 1987, 4,665,051 dated 12 May 1987 all to Nonnenmann alone or with another; U.S. Pat. No. 5,070,694 dated 10 Dec. 1991 to Whittenberger; International PCT Publication Numbers WO 89/10470 (EP412,086) and WO 89/10471 (EP 412,103) each filed 2 Nov. 1989, claiming a priority date of 25 Apr. 1988. The above International Publication numbers disclose methods and apparatus for increasing the internal resistance of the device by placing spaced discs in series, or electrically insulating intermediate layers. Another International PCT Publication is WO 90/12951 published 9 Apr. 1990 and claiming a priority date of 21 Apr. 1989, which seeks to improve axial strength by form locking layers of insulated plates. Another reference which seeks to improve axial strength is U.S. Pat. No. 5,055,275 dated 8 Oct. 1991 to Kannanian et al. Reference may also be had to International PCT Publication Number WO 92/13636 filed 29 Jan. 1992 claiming a priority date of 31 Jan. 1991. This application relates to a honeycomb body along an axis of which fluid can flow through a plurality of channels. The honeycomb has at least two discs in spaced relation to each other. According to the specification, there is at least one bar type support near the axis by which the discs are connected together and mutually supported. The invention is said to make possible design of the first disc for fast heating up through exhaust gas passing through or applied electrical current. The honeycomb body serves as a support for catalyst in the exhaust system of an internal combustion engine. Another reference is German Patent Application Number 4,102,890 A1 filed 31 Jan. 1991 and published 6 Aug. 1992. This application discloses a spirally wound corrugated and flat strip combination wherein the flat strip contains slots and perforations and is electrically heatable. The flat strips include a bridge between leading and trailing edge portions. Groups of such strips are separated by insulation means. The core is provided with a pair of circular retainer segments which are separated by insulation means. No end tabs are provided and the flat strip portions are integral. Another reference is U.S. Pat. No. 5,102,743 dated 7 Apr. 1992 to Maus et al. This patent discloses a honeycomb catalyst carrier body of round, oval or elliptical cross-section including a jacket tube, and a stack of at least partially structured sheet-metal layers intertwined in different directions in the jacket tube. The stack has a given length and a given width. At least one of the sheet metal layers has a greater thickness over at least part of one of the dimensions than others of the layers. Such at least one layer is formed of thicker metal or of a plurality of identically structured metal sheets in contiguous relation.
A common problem with prior devices has been their inability to survive severe automotive industry durability tests which are known as the Hot Shake Test and the Hot Cycling Test.
The Hot Shake Test involves oscillating (100 to 200 Hertz and 28 to 60 G inertial loading) the device in a vertical attitude at a high temperature (between 800.degree. and 950.degree. C.; 1472.degree. to 1742.degree. F., respectively) with exhaust gas from a running internal combustion engine simultaneously passing through the device. If the core of the catalytic device telescopes or displays separation of folding over of the leading or upstream edges of the foil leaves up to a predetermined time, e.g., 5 to 200 hours, the device is said to fail the test. Usually a device that lasts 5 hours will last 200 hours. Five hours is equivalent to 1.8 million cycles at 100 Hertz.
The Hot Cycling Test is performed with exhaust gas flowing at 800.degree. to 950.degree. C.; (1472.degree. to 1742.degree. F., respectively) and cycled to 120.degree. to 150.degree. C. once every 15 to 20 minutes for 300 hours. Telescoping of the core, or separation of the leading edges of the thin metal foil strips is considered a failure.
The Hot Shake Test and the Hot Cycling Test are hereinafter called "Hot Tests" and have proved very difficult to survive. Many efforts to provide a successful device have been either too costly or ineffective for a variety of reasons.
The structures of the present invention will survive these Hot Tests.
Early embodiments of electrically heatable catalytic converters were relatively large, especially in an axial direction, e.g., 7 to 10 or more inches long and up to 4.5 inches in diameter. These were inserted into an exhaust system as a replacement for the conventional catalytic converter now in common use, or in tandem relation with such conventional catalytic converter in the exhaust line. It was then found that an axially thin or "pancake" electrically heatable corrugated thin metal honeycomb monolith could be used in close tandem relation with the conventional catalytic converter.
It was later found that even better performance resulted from a "cascade" of converters, a low thermal inertia electrically heatable converter (EHC), followed by a medium thermal inertia converter, followed by a large thermal inertai main converter, all on the same axis of gas flow. Each converter had not only a different thermal inertia, but also a different geometrical cross-sectional area, or a different geometrical configuration, e.g., small circular, larger oval, to the final oval size and shape of a conventional unheated converter. This solution provided for fast, economical heating of the EHC. Heat generated from an oxidation reaction initiated in the EHC then heated the intermediate or "light-off" converter which in turn heated the large conventional catalytic converter.
It should be noted that the electrically heatable honeycomb acts to preheat the exhaust gas to its "light-off" temperature where, in the presence of catalyst, the pollutants are converted. Some conversion occurs in the electrically heatable device, and most Of the conversion occurs in the final catalytic converter section which is not normally electrically heated.
It has recently been found that a "pancake" electrically heatable device and a conventional multicellular metal monolith catalytic converter may be positioned together within a common housing to take advantage of the common diameter and/or geometric configuration (e.g., circular, oval or elliptical) in a cascading device, and having a shorter axial length than required in either a tandem relation or the prior cascade relation. These prior devices provide three units of differing thermal inertia. However, instead of a three-structural member cascade device, the improved structure enabled a two member cascade device. Reference may be had to the copending, commonly owned patent application of William a Whittenberger and Edward T. Woodruff entitled "Core Element Useful in a Combined Electrically Heatable and Light-Off Converter" filed 3 Feb. 1993 and given Ser. No. 08/013,516; and to copending, commonly owned patent application Ser. No. 08/066,887 filed 25 May 1993 by William A. Whittenberger entitled "Combined Electrically Heatable Converter Body."
The present invention is an improvement on the cascade devices of the preceding applications in that it enables easier manufacture and enables formation of both the electrically heatable portion and the light-off portion to be encased in a single housing. Instead of the back end, or trailing edge portion of the foil being allowed to "float" electrically, the back end of the foils of this invention are not grounded, and hence there is no opportunity for shorting out. Longer heating portions in the leading edge portion increase the resistance of the heating strip. It is also possible with the devices hereof to provide localized heating patterns as will be explained below. The dual purpose devices hereof may be backed up with a conventional catalytic converter of, for example, the commonly used ceramic type, the alumina pellet type, or the thin metal monolith honeycomb type. Each portion has a different thermal inertia, the electrically heatable portion being the lowest of the three. Thus, the advantages of the cascade effect for successive light-off may be utilized without encountering a number of the problems associated therewith. Avoiding substantial electrical heating of a major portion of the thin metal honeycomb effects a major saving in electrical power required. The devices hereof may be thought of as "integral" in that all of the thin sheet metal layers have the same axial length albeit different circumferential lengths, thereby providing improved durability in, for example, the Hot Tests.
In the following description reference will be made to "ferritic" stainless steel. A suitable ferritic stainless steel for use particularly in the engine exhaust applications hereof, is described in U.S. Pat. No. 4,414,023 dated 8 Nov. 1983 to Aggen. A specific ferritic stainless steel alloy useful herein contains 20% chromium, 5% aluminum, and from 0.002% to 0.05% of at least one rare earth metal selected from cerium, lanthanum, neodymium, yttrium, and praseodymium, or a mixture of two or more of such rare earth metals, balance iron and trace steel making impurities. A ferritic stainless steel of the foregoing analysis is commercially available from Alleghney Ludlum Steel Co. under the trademark "Alfa IV."
Another metal alloy especially useful herein is identified as Haynes 214 alloy which is also commercially available. This alloy and other nickeliferous alloys are described in U.S. Pat. No. 4,671,931 dated 9 June 1987 to Herchenroeder et al. These alloys are characterized by high resistance to oxidation. A specific example of an alloy useful herein contains 75% nickel, 16% chromium, 4.5% aluminum, 3% iron, optionally trace amounts of one or more rare earth metals except yttrium, 0.05% carbon and steel making impurities. Haynes 230 alloy, also useful herein, has a composition containing 22% chromium, 14% tungsten, 2% molybdenum, 0.10% carbon, and a trace amount of lanthanum, balance nickel. The ferritic stainless steel, and the Haynes alloys 214 and 230 are examples of high temperature resistive, Oxidation resistant (or corrosion resistant) metal alloys that are suitable for use in making thin metal strips for use in the converter bodies hereof, and particularly for making integral heater strips for the EHC portions. Suitable metals must be able to withstand "high" temperatures of 900.degree. C. to 1200.degree. C. (1652.degree. to 2012.degree. F.) over prolonged periods.
Other high temperature resistive, oxidation resistant metal alloys are known and may be used herein. For most applications, and particularly automotive applications, these alloys are used as "thin" metal strips, that is, having a thickness of from about 0.001" to about 0.005", and preferably from 0.0015" to about 0.003".
The features of the converters of the present invention are also advantageous in low temperature catalyzed applications, e.g., below zero Centigrade to room temperature and up to 500.degree. F. using metals such as copper, ordinary steels, zinc, silver, tin, chromium, aluminum, nickel, titanium, magnesium, molybdenum, etc. to form the thin metal foil strips hereof. In such applications heat may be abstracted from the foil strips instead of added. However, the generally more expensive high temperature resistive, oxidation resistant alloys may be used in such lower temperature applications if desired.
In the following description, reference may also be made to fibrous ceramic mat, woven ceramic tape, or fabrics, or insulation. Reference may be had to U.S. Pat. No. 3,795,524 dated 5 Mar. 1974 to Sowman, and to U.S. Pat. No. 3,916,057 dated 28 Oct. 1975 to Hatch, for formulations and manufacture of fibers useful in making tapes and mats which may be used herein. One such woven ceramic fiber material is currently available from 3-M Co. under the trademark "NEXTEL" 312 Woven Tape and is useful for insulation of thin metal strips or groups thereof. Ceramic fiber mat is commercially available also from 3-M Co. under the trademark "INTERAM". For most purposes, a coating insulation layer on the surface of the strips, for example and as later described, is preferred.